Residual fuel oils, also known as heavy or bunker fuel oils, are typically used as transportation fuel in marine applications and as burner fuel for power or heat generation purposes in industrial applications. Historically these fuel oils consist of the residue from distillation processes in crude oil refineries, including vacuum and cracking units. As such, they comprise complex mixtures of high molecular weight, high density compounds, with higher viscosity. They have a typical boiling range from about 350° C. to about 650° C.; and carbon numbers in the range from about C20 to C50 or above.
Critically, these residual fuel oils will almost inevitably contain high levels of organo-metallic, complex aromatic and hetero-species which remain behind as a residue of the distillation process. As such, on combustion, heavy fuel oils are significant sources of pollutants such as metals, soot and sulphur oxide species; and in their use, including marine applications, can represent a substantial environmental hazard in the case of spillage. Furthermore, in some sensitive direct heating applications (such as those in the food or pharmaceutical industries), the presence of sulphur, aromatics and metals in the fuel oil is highly undesirable because of the potential impact on product generation and purity.
These problems are all exacerbated in the current situation where the global supply of crude oils is shifting to lower qualities with concomitantly higher contents of sulphur, metals and other contaminants ending up in the residual fractions—resulting in crude-derived heavy fuel oils which are hence of considerable concern from both a health and environmental perspective.
In the marine environment, for example, current regulations have been introduced requiring the use of low-sulphur fuels in designated near-shore Emission Control Areas (ECA's). Whilst abatement technologies are a viable (if expensive) alternative; these regulations have typically required the use of middle distillate fuels in order to meet the requirement as these can be easily obtained with low sulphur content. Switching between distillate in ECA's and the more cost-effective residual fuel outside of these areas can cause significant technical problems on board ship. These are almost all the result of mismatch between the properties of middle distillate and heavy fuel oil such as viscosity and density, in complex systems which have been designed around the inherent properties of heavy fuel oil as discussed in “Special Report: Global marine fuel-switching to comply with sulphur emissions limits—problems and solutions”; John Liddy; Feb. 7, 2011; International Fuel Quality Center.
Crude-derived heavy fuel oils, whilst fulfilling a significant energy source requirement; are hence becoming more and more problematic in terms of the inherent pollutants and environmental impact associated with their use. Whilst it may be possible to substitute this fuel oil with cleaner middle distillate in certain applications, the property differences between these products renders this solution sub-optimal for many purposes. There is therefore a strong need for a suitable high quality, high performance, non-polluting replacement fuel that can be used in these types of applications.